It's a grey area, and quite risky. You can try to put your UA FFP number on the reservation and just flash your ANA card for benefits. However, every SAS agent you encounter (from check-in to lounge dragon) may try to proactively change your FFP back to ANA, and you'd have to keep checking and potentially change it back to UA yourself after lounge access, but before boarding. This is normally done through online check-in - not sure if the SAS website/app allow such maneuvers.